||Flow field control in marine fish larviculture tanks: lessons from groupers and bluefin tuna in Japan
Sakakura, Y. ,
Yamazaki, W. ,
Takakuwa, Y. ,
Sumida, T. ,
Takebe, T.Hagiwara, A.
521 , 2019-01-01 , Elsevier B.V.
Flow field in a larviculture tank is often based on the empirical assumptions by fish culturists and science-based information had been very limited. Therefore, we studied and reviewed research on the flow field dynamics in larviculture tanks of groupers and Pacific bluefin tuna, and compared them with results from our own and other studies. For radial symmetry tanks (cylindrical or octagonal), aeration by one aerator at the bottom of the tank forms vertical circulation. We quantified and visualized this flow at different aeration rates in a 1 kL tank and compared survival rates of seven-band grouper Epinephelus septemfasciatus and devil stinger Inimicus japonicus. The highest survival with the lowest surface tension-related death (STRD) was achieved when vertical flow velocity above an aerator was 8 cm/s (200 mL/min) for larvae of E. septemfasciatus, while better survival was observed at >8 cm/s in I. japonicus. Larviculture tank proportions also influence both flow field and performance of fish larvae. When tanks were set with the same water volume (100L) and aeration rate (50 mL/min) but different aspect ratios (AR: water depth/tank radius), survival of larvae for the above two species in a tank with AR >2.0 was found to be significantly higher with lower STRD. Flow field in a vertical cross-section of a tank changed from a single-pair vortex system to two-pair vortex systems as AR changed from 1.0 to 2.0. However, sinking syndrome, which causes high mortality by sinking of larvae to the tank bottom during darkness in some marine fishes (i.e., Pacific bluefin tuna Thunnus orientalis), cannot be prevented with the above conventional flow field management. Two flow field control methods have been proposed in order to prevent sinking syndrome. One is increasing the aeration rate at darkness for vertical mixing of the rearing water. The other is a ‘water pump system’, where a water pump was put in a small net cage with fine mesh connected to a drain at the center of the tank, and water from the water pump was discharged via a cross-shape pipe on the tank bottom. Both were found to be effective to prevent sinking syndrome. Flow fields in the conical-cylindrical tanks and the rectangular tanks, which are used widely and have many different proportions, have not been well quantified yet. We have expanded our approach investigating the flow patterns in rectangular tanks by three dimensional two-phase aerated flow simulations, and found that velocity fields were different from those of cylindrical tanks.